1. Field of the Invention
The present invention relates to aging for optical fiber gratings used for filters, multi/demultiplexers, dispersion-compensators, and the like in optical fiber networks.
2. Related Background Art
An optical fiber grating is an optical fiber having a region therein (mostly in its core portion) provided with a periodic change of refractive index along the longitudinal direction of the optical fiber. The region where the refractive index changes, which is known as the grating, can transmit or reflect propagated light according to its wavelength. Consequently, the optical fiber grating is applied to various kinds of optical elements such as filter, multi/demultiplexer, dispersion-compensator, and the like.
As shown in FIG. 1, a grating 20 is often formed by a method comprising the steps of preparing a silica-based optical fiber 10 in which GeO.sub.2 (germanium dioxide) is added to at least its core region, irradiating this optical fiber 10 with an interference fringe formed by light 30 having a predetermined wavelength, and generating a change in refractive index corresponding to an optical energy intensity distribution of this interference fringe. Since the optical fiber 10 is usually coated with a plastic layer (not shown), a part of the coating is removed, and thus exposed part of the optical fiber 10 is irradiated with the light 30. In FIG. 1, numeral 22 indicates parts where a larger amount of increase in refractive index is induced upon the irradiation, whereas numeral 24 indicates parts where a smaller amount of increase in refractive index is induced. The grating 20 is a region where the parts 22 and 24 are alternatively and periodically disposed along the longitudinal direction of the optical fiber 10.
It has been considered that the irradiation with a certain wavelength of light generates Ge-defects in the GeO.sub.2 -doped portion in the silica-based optical fiber, thereby causing the change in refractive index. It has also been known that the amount of Ge-defects changes with time, whereby characteristics of the optical fiber grating deteriorate over time. In view of these points, there have been proposed techniques which perform accelerated aging for an optical fiber grating immediately after its manufacture to sufficiently suppress its aged deterioration upon operation in the market. Examples of such techniques are disclosed in U.S. Pat. Nos. 5,287,427 and 5,620,496 which are incorporated herein by reference.
In the technique disclosed in U.S. Pat. No. 5,620,496, normalized refractive index difference .eta. is supposed to be represented by the following relational expression: ##EQU1## where t represents time, and C and .alpha. are functions of temperature. The normalized refractive index difference .eta. is a value of the refractive index difference of a grating when time t has elapsed from a predetermined point of time (i.e., reference time) after formation of the grating, and this value is normalized with respect to the refractive index difference of the grating at this point of time. Namely, .eta.=(refractive index difference at t after the reference time)/(refractive index difference at the reference time). In the technique disclosed in the above patent, the time immediately after grating formation is adopted as the reference time. The refractive index difference refers to the difference between the maximum and minimum values of refractive index in the grating.
In the conventional techniques, from the fact that .eta. changes more rapidly as the temperature is higher, the optical fiber grating is heat-treated in an environment at a temperature higher than its operating temperature to perform the accelerated aging, in order to suppress the deterioration upon its operation.